The present invention relates generally to floating platform systems for testing and producing hydrocarbon formations found in offshore waters. More particularly, the invention relates to a method and system for achieving hydrostatic stability of a floating structure while ballasting during installation of the structure.
The exploration for oil and gas deposits in offshore waters, and recovery of the oil and gas therefrom is very expensive. Various methods and offshore production systems have been utilized to locate and recover offshore oil and gas deposits. Exploration and production systems such as converted Mobile Offshore Drilling Units (“MODU”), Tendon Leg Platforms (TLP) and other floating structures typically used in offshore waters are very expensive to manufacture and install.
Installation of an offshore platform, such as a TLP, may require that the platform hull be wet towed to the installation site. The hull of a single column TLP comprises the central column and a plurality of pontoons extending radially outwardly from the central column. The hull of a single column TLP is quite stable floating on the pontoons because of the large water plane area provided by the central column and pontoons. During installation, the hull is ballasted down for connection to a plurality of tendons which anchor the hull to the seabed. As the hull is ballasted down, the hydrostatic stability of the hull decreases significantly after the pontoons submerge. To reduce stability problems during installation, the hull is typically installed without the deck and with a stabilizing upward force applied at the top of the hull as it is ballasted down to connect with the pre-installed tendons. This stabilizing force is typically supplied by a derrick barge. After the hull is lowered, connected to tendons and deballasted, the system is very stable. The deck may then be lifted using the derrick barge and safely set on the platform hull.
For some offshore platform installations, it may be very advantageous commercially to utilize an installation alternative that eliminates the need for the expensive derrick barge during hull and deck installation. One such installation method, would for example, include installing the deck on the hull in a less exposed, shallower water location with less expensive lifting means, wet-towing the hull-deck assembly to the installation site, and safely lowering the hull-deck assembly and connecting it to pre-installed tendons without using a derrick barge.
It is therefore an object of the present invention to provide a method of installing an offshore floating platform without using a derrick barge.
It is a further object of the present invention to provide a method of installing a floating platform including the steps of assembling the complete hull, deck and most production equipment at or near the fabrication site; wet towing the assembled platform to the installation site; and ballasting down the assembled platform to tendon connection draft without a derrick barge.
It is a further object of the present invention to provide a method of installing a floating platform by providing hydrostatic stability for the hull of the platform at all drafts through the use of supplemental buoyancy.